The optics for lithography systems for the extended ultraviolet (EUV) into the x-ray regime (short wavelength radiation) are generally reflective. Even the lithography masks are reflective optics.
The short wavelength lithography masks include multilayer (ML) Bragg mirror stacks of many bilayers of high and low refractive index material. EUV masks, for example, usually have 40 bilayers of molybdenum and silicon, which have been successively coated onto a substrate. In general, a Bragg reflection of around 70% at 13.5 nanometer (nm) wavelength is targeted.
The short wavelength lithography masks have additional layers. On top of the ML mirror stack is a capping layer for environmental protection. In addition, this capping layer acts as an etch stop during mask fabrication. As a result, the capping layer should have a low EUV absorption. The stack is finalized by an optional buffer layer (e.g. SiO2). Finally, an anti-reflecting absorber layer (e.g. TaN) is patterned to define the dark and bright (reflective) features of the mask.
Because of the complex multilayer structure, these ML lithography masks are subject to defects known as multilayer (ML) defects (also called buried defects). These ML defects can come from the low thermal expansion material (LTEM) substrate in the form of pits, bumps or scratches that are created on the substrate surface from the chemical mechanical polish (CMP) and cleaning processes used to prepare the substrate prior to the deposition of the mirror stack. In fact, small substrate defects, e.g., below ˜20 nm, are considered process inherent during CMP and cleaning ML defects can also arise during the ML deposition process. The complex multilayer, typically consisting of 80 or 100 alternating layers of Si and Mo, is deposited on this substrate followed by a Ru capping layer. Ion beam deposition (IBD) is normally used for the ML deposition steps and over half of all killer blank defects can be traced to this deposition step.
These ML defects are not easily detectable or capable of characterization by current inspection tools. Yet the defects present on the LTEM substrate or arising in the ML layers propagate through the ML mirror stack and will nevertheless print during EUV exposure if they affect reflective features of the final patterned photomask.